1. Field of the Invention
This invention relates to modulation or control of an electrical current which is dependent on the resistance of a sensing element where the two wires which carry power to the sensing circuitry are also used as the signal current transmission means. The measurement is one in which direct current power is supplied to remotely located sensing and current modifying circuitry which acts to control the total current flow proportional to a measurand.
2. Description of the Prior Art
The problem of conversion of a measurand (where "measurand" refers to the quantity or physical variable being measured) to an electrical signal and subsequent transmission of that signal to recorder and control equipment which may be located some distance away has been handled in many ways in the past. In general four-wire systems have been used where power is supplied via two of the wires and a voltage signal is transmitted via the other two wires. One of the voltage signal leads may be common to one of the power leads for some of these systems. Such a system typically requires use of an amplifier and/or other signal conditioning equipment at the point of measurement in order to supply an accurate signal representative of the measurand. The advantages of using the same two wires for power supply and information transmission has long been recognized and various designs of transmitting equipment have evolved. The prior art approach for force transducers where direct current power and signals are required has been to supply sufficient current to rebalance the force being measured by current through an electromagnetic arrangement. A small amount of current is routed through a null sensing circuit and amplifier which in turn controls the main current supply to the force balance coil. Such circuitry is widely used in industrial pressure measurements and an example is described in U.S. Pat. No. 3,274,833.
For temperature sensing transmitters self-balancing circuits using a motor driven potentiometer have commonly been used. Examples of various designs which have been developed in the past are given in the chapter titled "Measuring and Transmission Methods" of the book Handbook of Applied Instrumentation, McGraw-Hill Book Co., 1964, Library of Congress Catalog Card No. 62-21926. Those various designs all employ electro-mechanical elements of one type or another, to achieve self-balancing circuitry operating from a resistance signal or thermocouple signal. In almost all cases a two phase motor is relied on to provide adjustment of a potentiometer or variable condenser to achieve a balance condition. In one example, the current from a thermocouple passes through the field of a permanent magnet deflecting a beam against a calibrating spring. Beam deflection is sensed by other circuitry which supplies a high level, direct current signal suitable for transmitting to a recorder. A portion of the signal is shunted through a feedback coil which opposes the force caused by the thermocouple current thereby maintaining a balanced condition.
Since these previous designs have all required electromechanical devices they have the disadvantages of rather slow response, limited life associated with a loss of resolution where frictional contacts are involved, and poor performance under adverse environments such as wide variations in temperature, excessive humidity and dust.